Under this system, there would be no real incentive to sell items on the Market Board. If you can't save your money (for high-level crafted gear, housing, rare minions that you don't want to farm yourself, whatever cool things might be coming in future expansions) even with your retainer, there's no reason to make money. Especially if, as you propose, gil earned from questing and the like is still similar to what it is now. If you can cap gil easily, where's the incentive for earning it in the first place?

I also agree with the others - RMT will still happen, they'll just find things other than gil to sell.